Value Depends On Previous Values

I am analysing data to determine buy or sell transaction signals. The problem is that the method can and does produce frequent signals that are merely confirmation rather than new information.

Therefore, I need to select a signal, eg a "buy" signal where the previous transaction signal was the opposite, eg a "sell" signal.

In the attached sheet N10 is a valid signal as the previous signal was a "sell" signal. However, N14 is not valid as the previous signal was also a "buy" signal.

Currently, I am using a simple formulae to determine the signals, e.g. =IF(J26>0,J26,0). Hoevever, I need to modify this by looking back to confirm that a value >0 occurred first in column K rather than column J. If the value >0 occurred first in column J then I need to ignore the buy signal.

I am not sure if the formulae can be modified or if a macro is needed.